1. Field of the Invention
The present invention relates to the manufacture of porous electrodes for energy storage devices and energy conversion devices by chemical and thermal spray techniques. In particular, this invention relates to the fabrication of thin film oxide and non-oxide electrodes by thermal spray. Such manufacture advantageously uses continuous processes suitable for high-volume electrode production.
2. Description of the Related Art
Energy storage devices, such as batteries and supercapacitors, and energy conversion devices, such as fuel cells and thermoelectrics, both require electrodes comprising an active material for the energy storage, conversion, and/or release processes. Each year, billions of dollars are spent on both primary and rechargeable batteries for use in applications ranging from small batteries for portable electronics and communications equipment, to larger batteries used for automobiles and uninterruptible power supplies (UPS). Many of the industrial manufacturing processes associated with the fabrication of the electrodes containing active material (faradaic) are based on batch processes, often incorporating labor-intensive hand operations. There exists a critical need to develop continuous processes for electrode manufacture that enable the production of low-cost electrodes for both energy storage and energy conversion devices.
There is especially a need for efficient manufacture of thin film electrodes (e.g. 10 mil or less), where conventional pressing techniques are inappropriate for disk electrodes with diameter in excess of 2 inches in the absence of a supporting substrate. Thin film electrodes have been fabricated by various techniques, including spray pyrolysis and chemical vapor deposition (CVD). Spray pyrolysis is used in the preparation of thin films comprising metal oxides. In spray pyrolysis, a negatively-charged substrate with a heating element to control the temperature is provided, and a precursor solution with the proper molar ratio is forced to flow through a positively charged nozzle onto the negatively charged substrate. The spray droplets tend to move to the hot substrate, primarily due to electrostatic attraction, and pyrolysis takes place at or near the surface of the substrate. This technique has been used to fabricate electrodes comprising LiCoO2, LiMn2O4, yttria stabilized zirconia (YSZ).
Thin film electrodes have also been previously fabricated by chemical vapor deposition (CVD) and related techniques. A typical CVD process involves the steps of vaporizing precursors to the vacuum chamber; triggering reaction of the vaporized precursors; and depositing the reaction product onto the surface of substrate. This basic process has been used to fabricate electrodes comprising MoS2 by conventional CVD, ZrO2-TiO2-Y2O3 by laser CVD (wherein the laser is the heat source of the substrate and reaction activator), and TiS2 by plasma CVD.
Thin film electrodes have also been prepared by sol-gel methods (CeO2-TiO2 electrodes), electrochemical method (amorphous MnO2 electrodes), and molecular beam deposition (xcex3-In2Se3). Most, if not all of the above-described processes have been limited to small-scale electrode production, and are further ill-suited to adaptation to a continuous process suitable for low-cost, high volume production.
Recently, fabrication of electrodes by thermal spray has been disclosed by U.S. Pat. No. 5,716,422 to Muffoletto et al., which is incorporated herein by reference. U.S. Pat. No. 5,716,422 teaches the use of a variety of thermal spraying processes for depositing an electrochemically active material onto a substrate, resulting in a thin film electrode. Suitable spraying techniques include chemical combustion spraying processes, for example powder flame spraying, and electric heating spraying processes, for example plasma spraying. Muffoletto et al.""s preferred electrochemically active materials include metals, metal oxides, mixed metal oxides, metal sulfides and carbonaceous compounds and mixtures thereof. More particularly, the use of copper oxide, cobalt oxide, chromium oxide iron sulfide and iron disulfide is disclosed.
A significant drawback of the thermal spraying processes results from the thermal instability of some of the electrochemically active materials, particularly iron disulfide (xe2x80x9cpyritexe2x80x9d). Pyrite is thermally unstable, decomposing to FeS at about 550xc2x0 C., which is much cooler than the flame temperature of plasmal spray. Although certain well-known techniques can provide lower flame temperature, the oxidized nature of the flame (the flame consists of propylene and oxygen) prevents the possibility of its application in the spraying of pyrite.
There accordingly remains a need for methods for producing electrodes using thermal spraying and certain preferred electrochemically active materials while avoiding thermal decomposition of said electrochemically active materials. Further, there exists a need for a thermal spraying process wherein the electrochemically active materials comprise nanostructured materials.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the method of the present invention, wherein thermal spray of active material feedstocks is used to fabricate porous electrodes for energy storage devices and energy conversion devices. Active material feedstocks for thermal spray are readily available by chemical synthesis in aqueous solution at low temperature ( less than 90xc2x0 C.). In an advantageous feature of the present invention, the active material feedstocks undergo a reprocessing step whereby they are uniformly coated with sulfur prior to thermal spray. The sulfur coating prevents thermal decomposition of the active materials during the spraying process. Thermal spray methods function with a wide variety of active material feedstocks, and are readily adaptable to continuous manufacturing processes. In another advantageous feature of the present invention, the active material feedstock comprises nanostructured materials, which after thermal spray results in electrodes having nanostructured active materials.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.